The invention relates to an apparatus for transparency scanning of continuous-tone or half-tone originals in electronic reproduction technology, preferably for flat bed scanners.
When, in tranparency scanning of an image original (usually a silver film), such an original is transilluminated with highly directional light, then after passage of the light, different measured values for the optical density result than would have derived if the original has been transilluminated by diffuse light. The values for highly directed light seem shifted toward high densities. This phenomenon known as the Callier effect arises due to light scatter at the silver grains in the photographic layer and essentially depends on their size and distribution.
In electronic reproduction technology, this effect appears in an image-falsifying way particularly when, in what are referred to as flat bed scanners, the planarly disposed original is scanned by a highly focused light ray moved transversely relative to the feed direction of the original (referred to as "flying spot scanning"). Lasers are employed as light sources in modern flat bed scanners. The narrow light ray of these lasers, after expansion, is guided onto a rotating polygonal mirror and, after correction of the errors which arise due to deflection by means of what is referred to as a scanner lens, is focused by this scanner lens in the plane of the original. As a result of the polygonal mirror, the beam executes a swinging motion over the original which is slowly advanced at right angles relative to the plane of swinging, so that this original is scanned line-by-line. The light which has passed through the original and whose intensity is dependent on the optical density of the image point respectively impinged by the light ray is supplied to a photoelectrical transducer in a suitable way along the scan line. This transducer converts the optical density values of the original into serial, electrical signals.
Without further techniques, the density values in this type of scanning would be falsified by the afore-mentioned Callier effect, and this would have a disturbing appearance in the re-recording.
U.S. Pat. No. 4,453,180 discloses a light pick-up device for transparency scanning wherein an inwardly partially mirrored, partially diffusely reflecting tube is attached below the original parallel to the scan line. This tube has a longitudinal slot parallel to the scan line through which the light transilluminating the original enters. The end faces of a cross-sectional transducer formed of a plurality of optical fibers are arranged offset at a right angle relative to the light entry axis.
Apart from the extraordinary low efficiency, the disadvantage of this apparatus is that the mount for the original to be scanned is always formed of a glass plate which must have a considerable thickness for stability reasons. The light entry slot is at a distance from the original which amounts to at least this glass thickness. On the other hand, the light entry slot must be narrow as possible so that the arrangement can function with reasonable efficiency. What results therefrom is that the entry aperture for the scatter lobe is so greatly constricted that the disturbing influence of the Callier effect can only be imperfectly influenced therewith.
Even given moderate densities, the aperture angle of the scatter lobe of the light emerging from the original is significantly greater than the entry angle into the tube limited by the edges of the slot. The light scatter covered by the slot edges can thus not contribute to the measurement.